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I have a question about Linear Rails

Posted: Fri Sep 06, 2019 7:28 pm
by Dallas
I'm upgrading my z-axis -- a poor man's version of the HDZ. I'll share the details in a couple of weeks (assuming it works. :lol: )...

I have a question for the community. I bought really, really cheap MGN12 linear rails from Banggood: ... rehouse=CN

I got the 250mm long ones. The first set arrived and I notices a slight bend in the rail. If you put the rails back to back, there's maybe a 1-2mm gap at the ends; they curve away from each other.

These are pretty beefy pieces of steel so I don't think they got bent during transport. I figured that went I socked them down to the baseplate, it would take the bend out so no big deal.

All by themselves, the trucks glide on the rails like a hot iron on a frozen lake (even with the bend). When I tighten down my cross plate to the trucks, the trucks start to bind. Now the cross plate is flat so I'm not buckling the trucks as far as I know. If I loosen the screws ever so slightly, I can get the trucks to glide easily again.

I wondered if the bend was causing the issue. A dead-flat truck wouldn't want to glide on a curved rail. So I bought another set from Banggood. (They are cheap; it was a low risk investment.) The second set arrived yesterday but they also are slightly curved; just like the first set. I haven't had time to install them to see if the tight-mounting-screws-cause-binding issue resurfaces.

I'm not complaining about the quality of the linear rails. You get what you pay for.

My question is: Are linear rails supposed to be 'pre-sprung'. Perhaps that bend is normal... Is there a reason that the trucks would bind when something is mounted to them very tight?

I have no experience with linear rails and would love to hear from those who do.

(As an aside: I bought a dirt cheap ballscrew off eBay and was pleasantly surprised with the quality. It will be more than adequate for my needs. So dirt cheap isn't always that bad.)